A radio terminal, in order to communicate with a radio base station, needs to be located within a range (a service area) where radio waves from the radio base station reach. However, a mountainous area and a city with many high-rise buildings have areas where the radio waves scarcely reach due to numerous obstructions. Also, there are many areas where the radio waves from the radio base station installed outdoors may not reach (for example, the inside of a building or underground). Especially, high-speed radio communication schemes such as WiMAX (registered trademark) (WiMAX: Worldwide Interoperability for Microwave Access) standardized based on IEEE standard 802.16e and the like use a frequency band of 2.5 GHz or higher and, since radio waves in such a high frequency band are more likely to travel straight rather than to travel around obstacles, the radio waves are strongly affected by the obstacles. In order to cover such an area where the radio waves may not reach, a radio relay apparatus (repeater) for relaying the radio waves between the radio base station and the radio terminal is needed.
The radio relay device, while having an advantage to allow expansion of the service area, has a disadvantage that the radio waves emitted therefrom causes interference with other radio waves. As an example of interference caused by the radio relay apparatus, there is mutual interference between a donor node in the radio relay apparatus (a base-station-side block) and a service node (a terminal-side block). Here, the donor node is a block for transmitting and receiving data with the radio base station, and the service node is a block for transmitting and receiving the data with the radio terminal.
FIG. 8 illustrates a state of, in the radio communication system employing the TDD scheme, the mutual interference between the donor node and the service node when the radio relay apparatus operates at normal transmission/reception timings. The TDD scheme is a method of performing fast switchover between transmission and reception by finely dividing time.
At normal transmission/reception timings (hereinafter, referred to as “normal timings”), timings for transmission and reception of the radio base station and those of the service node of the radio relay apparatus are synchronized. That is, in a first period, the radio base station transmits downlink (DL) data to the donor node of the radio relay apparatus, and the service node of the radio relay apparatus transmits the downlink data to the radio terminal. In a second period, also, the radio base station receives uplink (UL) data from the donor node of the radio relay apparatus, and the service node of the radio relay apparatus receives the uplink data from the radio terminal.
When the radio relay apparatus is operating at the normal timings, in the first period the donor node receiving a downlink signal from the radio base station is interfered by a downlink signal transmitted from the service node to the radio terminal. Also, in the second period, the service node receiving an uplink signal from the radio terminal is interfered by an uplink signal transmitted from the donor node to the radio base station.
As a method of reducing the mutual interference during operation at the normal timings, there is suggested a method of operating the radio relay apparatus at transmission/reception timings illustrated in FIG. 9 (for example, see Patent Document 1). This method inverts the normal transmission/reception timings of the service node (hereinafter, referred to as “inversion timings”).
As described above, by synchronizing transmission and reception of the donor node and the service node of the radio relay apparatus, the mutual interference between the donor node and the service node, which is an issue of the operation at the normal timings, may be eliminated.